1. Field of the Invention
The invention relates to luminescent lighting systems and luminaires for capturing and directing light emanating from large planer light sources.
2. Description of the Prior Art
Fluorescent tubes produce light largely by conversion of ultraviolet radiations excited by electrical arc discharges through a low pressure gaseous medium within the tube containing mercury vapor into visible light through a process called photoluminescence, i.e. a nonthermal emission of electromagnetic radiation by materials called "phophors" upon excitation or absorption energy from higher energy (ultraviolet) radiations. The absorption and re-radiation of the light at the longer wavelengths by the phosphors is variously described as fluorescence and phosphorescence. The phosphors are typically high purity crystalline compounds which are deposited onto the interior walls of the tube. [See IES Lighting Handbook 5th Ed. 1972, pp 2-8, 2-9.]
By convention fluorescence has been and is defined as the process of emission of electromagnetic radiation by a substance as a consequence of the adsorption of energy from radiation, provided that the emission continues only so long as the stimulus producing it is maintained, i.e., a luminescence which ceases within about 10 nanoseconds after excitation stops. Phosphorescence has been and is defined as luminescence that is delayed more than 10 nanoseconds after excitation stops. [See Van Nostrand's Scientific Encyclopedia 7th Ed. pp. 1194, 1737 & 2189-90.]
It is also well known that the gaseous arc discharges in fluorescent lamp tubes are maintained by pulses of electrical current propagating back and forth between electrodes at the respective ends of the tubes supplied by ballast power sources. Typically, the current pulse frequency is twice the complete cycle frequency of the ballast power source, i.e., a current pulse propagating in one direction through the tube in the first half cycle and the opposite direction the second half cycle. [IES Lighting Handbook 5th Ed. 1972, pp 2-7/2-9 8-27/8-30; IES Lighting Handbook, Reference Vol. (1984) pp. 8-19/8-39 at 8-30; U.S. Pat. No. 4,467,247, Hammer; and U.S. Pat. No. 4,525,649, Knoll.]
Before 1988 light from "fluorescent" light fixtures generally was never thought to be suitable illumination for film, television or video productions, primarily because of the periodicity in the generated light output. For example, ordinary 60 Hz fluorescent lighting fixtures typically produced green `beating effects` in resulting television and motion picture imagery due to the color of delayed phosphorescence emissions from the phosphors. And, while the pulsing light flashes of the typical 60 Hz fluorescent lighting fixture (at the rate of 120 flashes per second) are not noticeable to the human eye, they have stroboscopic effects when multiple frames images are utilized to record and show motion.
In particular, per conventions adopted as standards in the television and video industries, electronic scanning cameras successively analyze or synthesize (sample) the light values of picture elements or pixels constituting the picture area according to a pre-determined method. The concept of picture elements or pixels as that term is used in the television arts, originated in 1884 with the Nipkow Mechanical-Optical Scanning System. [See Chapter 3 of the Television and Audio Handbook, by K. Blair Benson & Jerry Whittaker (1990), pp. 3.2-3.8. See also Television Engineering Handbook compiled by K. Blair Benson, published in 1986 by McGraw-Hill, Inc. at pages 4.1-4.9 for a more sophisticated (mathematical) treatment of the concept and theory of picture elements or pixels.] And, while it has not been appreciated until recently, it is generally known that a correspondence between space and time exists in film, television and video media which permit a "pixel's size" to be defined as either a "certain fraction of a scan line", or as "a duration in time". [Television Engineering Handbook (supra), Chapter 19, entitled "Digital Video Effects" at pages 19.8-19.9]
For example, the interlacing standards adopted by the television industry in the United States of America, comprises 30 frames/sec where each frame consists of 525 pixel lines. Each frame is further broken down into two parts or fields of even and odd number lines of 262.5 lines to provide a repetition rate of 60 fields/second (60 Hz). This repetition rate matches the U.S.A. standard AC power line frequency and thus mitigates the effect of `hum` due to imperfect power supply filtering. `Hum` causes vertically moving patterns to pass through the image. (For color the repetition rate is slightly reduce to 59.94 Hz which means that a `hum` interference pattern typically propagates vertically through the reproduced image at a rate of 0.06 Hz.)
Accordingly, if it is assumed that each line consists of 512 pixels or picture elements, then under the U.S. convention, electronic scanning cameras scan (and television sets project) at rate of approximately 15,750 lines/second or 8,064,500 pixels/second. Inverting these numbers, the duration of the camera's sampling per line and per pixel is approximately 63.5 microseconds/line and approximatly 124 nanoseconds/pixel, respectively [See Television Electronics: Theory and Servicing 8th Ed. (1983), pp. 71-72 for a rudimentary discussion of the timing relationships of lines, and pixels.]
Periodicity in illuminating lighting when images are reproduced by television/video has an effect analogous to `hum` interference patterns attributable to an unsynchronized power supply, only in this case, the patterns move horizontally rather than vertically through the recreated image. For example, if a ballast operating at 31.5 kHz is used to drive a luminescent system to produce 4 distinct flashes per scan line (63,000 flashes/sec) there would be 4 `brighter` vertically oriented bands in the projected image corresponding to the rise and fall times of the particular flashes. The periodicity of the flashes relative to the trace and retrace rates of the rectangular raster, may cause the bands to incline diagonally across the screen.
The Applicant discovered prior to 1988, that it was possible to "tailor" both the color/chromacity and duration of luminescent light emission to produce a sustained luminescent light emission suitable for television and video tape productions. [See parent application, Serial No. 07/177,099, filed by the applicant, Paul D. Costa in the United States of America on Apr. 4, 1988.]
Another perceived disadvantage of luminescent (fluorescent) illumination systems in the television, video film production industries has been the bulk or size of the fixtures. Typically, the fixtures mount long tubes in a spaced parallel flat arrays, a convex cylindrical arrays, or a concave cylindrical arrays. Light emanating from such tube arrays is typically directed perpendicularly outwardly from the array. This means that the effective (light emitting) apertures of such fixtures are quite large, in most cases, subtending solid angles greater than that occupied by the array of tubes. The large effective apertures of such fixtures have generally frustrated efforts to direct and shape the emanating light into beams and spots in the manner that light emanating from conventional incandescent (point) light sources can be directed and shaped.
Accordingly, luminescent (fluorescent) illumination systems have conventionally only been thought of as secondary illumination sources for television, video and film production, useful, for example, to illuminate large backgrounds areas, (U.S. Pat. No. 3,733,480, Glenn, Jr.), and to `balance` existing fluorescent light illumination at a particular locations, (U.S. Pat. No. 4,728,428, Lowell).
Other disadvantages of existing luminescent (fluorescent) illumination fixtures relate to fact that each tube effectively `shadows` the light output from its back surface reflected back toward the tube or through the tube plane by a typical reflector. Accordingly, there is a resulting loss of light intensity or (flux density). And in fact, in prior art luminescent (fluorescent) lighting fixtures and luminaires, the brightest or most intense (maximum light flux) light is typically measured as emanating from the facing front surface of the light tube. [See U.S. Pat. Nos. 4,602,448, Grove, 3,733,480, Glenn, Jr., and 2,264,141, Nemeroff.]